1. Field of the Invention:
This invention relates to an impeder for an electric resistance tube welding process for welding by heating opposite edge portions of a strip-skelp continuously formed into a tubular shape or a spirally tubular shape.
2. Description of the Prior Art:
In an electric resistance tube welding process, e.g., a high frequency induction tube welding process, the welding portion is constituted as shown in FIG. 1 of the accompanying drawings. That is, a skelp 1 formed into a tubular shape (hereinafter, referred to as a tubular skelp) is heated at the opposite edge portions by the action of eddy current induced in the tubular skelp 1 by passing a high frequency electric current through a coil 2 and the thus heated skelp is welded under pressure by means of squeeze rolls 4. An impeder used for improving welding energy utilization efficiency by concentrating the eddy current in the opposite edge portions is composed of an impeder casing 5 made of an insulating material and a core 6 made of a ferromagnetic material, and a mandrel pipe 7 connected to said impeder casing 5, and the core 6 is continuously cooled by cooling water supplied from said mandrel pipe 7.
The purpose of using an impeder is to improve the welding energy utilization efficiency of the electric resistance tube welding process. The performance of the impeder is determined by the characteristics of the core, which must have high magnetic permeability and high saturation flux density. That is, for improving the welding energy utilization efficiency in the electric resistance tube welding process, it is necessary to pass a large amount of magnetic flux through the core of the impeder.
The magnetic flux (.phi.) passing through the core is the product of the magnetic flux density (B) and the transverse cross-sectional area (A) of the core. The amount of the magnetic flux is shown by the following equation (1) EQU .phi.=B.times.A (1)
In this specification what is meant by the transverse cross-sectional area of the core or a laminated metal body is the area of the cross section taken in the direction perpendicular to the longitudinal direction of the impeder.
In conventional impeders, a ferrite core consisting of a magnetic oxide prepared by sintering a powdered oxide has been used.
In the case of the electric resistance tube welding process, the magnetic flux density (B) of the ferrite core is liable to saturate since the welding electric current is large and has a high frequency so that, particularly if the diameter of the tube is small, the transverse cross-sectional area (A) of the ferrite core becomes small and hence the magnetic flux density of the ferrite core approaches saturation at a relatively small welding electric current. In this case, as is clear from the above equation (1), the magnetic flux (.phi.) if the core approaches saturation and the welding energy utilization efficiency declines.
As a countermeasure to this problem, ferrite core makers have developed improved core materials with increased saturation flux density (B.sub.s '). Although B.sub.s ' values of about 0.5 tesla have been obtained, the degree of improvement is not yet satisfactory. There are core materials having a saturation flux density higher than that of the ferrite core, for example metallic magnetic materials such as silicon steel, low carbon steel, Permalloy (trade name), amorphous metals, etc., and a metallic magnetic material rolled to a thickness of 0.3-0.35 mm and heat-treated has been used as an iron core for transformers etc. for use with ordinary line current (50-60 Hz).
In the case of the electric resistance tube welding process, since the welding electric current used in the process is a large electric current having a high frequency (10-500 KHz), the impeder is exposed to a high magnetic flux density having a high frequency and thus a large eddy current is generated in the core which increases the temperature thereof. Also, since the eddy current is generated in proportion to the square of the magnetic flux density and the square of the frequency, the amount of heat generated by the core in the electric resistance tube welding process is very much larger than that of cores used at line frequency. Therefore, in the case of a metallic magnetic materials generating a large amount of heat by an eddy current, it is impossible to keep the temperature of the core of the impeder below the Curie point by cooling it with cooling water, which is the usual cooling method used industrially. Thus, the core loses its ferromagnetism and cannot be used as the core of an impeder. This is the reason that a ferrite core generating a small amount of heat by the eddy current in the high frequency region is used in the electric resistance tube welding process.